1. Field of the Invention
The present invention relates generally to reducing the standby power consumption of electronic devices. More particularly, the present invention relates to a circuit adapted to minimize standby power in a power supply system.
2. Background
Nowadays, most household electrical appliances are powered by DC (Direct Current) power, which is generally generated by power converting devices called “power adapters”. The power adapter inputs Power-frequency AC (Alternating Current) mains supply, which is generally 220V or 110V, and outputs low-voltage DC power to the household electrical appliances. Within the power adapters, the switch mode power-supply (SMPS) technology is applied; alternatively the Power-frequency transformer is applied to convert an AC mains supply to a low-voltage DC power supply. The power adapter using the Power-frequency transformer is thereinafter referred to as Power-frequency adapter. Generally, the power adapters are turned into standby mode when the electrical devices being powered are in a “sleep” mode or turned off. According to the present arts, the energy consumption of the Power-frequency adapter is often high, namely that its standby power consumption is high.
Conventional Power-frequency adapter often includes a Power-frequency transformer for lowering the voltage, a bridge rectifier, a filtering electrolytic capacitor, and an enclosure. The demand for them is huge and the competition is very keen. The Power-frequency transformer is often made from copper and silicon steel sheet, and has the disadvantages like big volume and heavy weight, but it also has the advantages such as high insulation resistance (easily achieved), good dielectric strength, good surge impact resistance, easy maintenance, and less electromagnetic radiation pollution, etc. Therefore, Power-frequency adapter still has a promising future, and its advantageous features can solve the problems that are very difficult to be overcame in the switch mode power supply technology.
In order to save the energies and protect the environment, many countries have issued successively the laws and regulations on energy-saving. For instance, the United States adopts the “Energy Star” Certification, which, with the help of the government of the United States, is aimed at promoting the environmental protection sense of business and individuals by certification of high-efficiency and energy-saving. After selecting and passing the test procedures strictly worked out by Environmental Protection Agency (EPA), the manufacturers of power adapters can gain the certificate of “Energy Star”, which means that the power adapters of the manufacturers will be allowed to be exported to some countries and areas. Thus the “Energy Star” certification is very important. China Certification Center for Energy Conservation Product as a partner of EPA also works out the certification standard similar to “Energy Star”.
The “Energy Star” Certification strictly specify the lower limit of energy utilization efficiency of power adapter at work mode, as well as the upper limit of energy consumption in standby mode. Furthermore it keeps pace with the times. For instance, it specifies the upper limit of standby power consumption of power adapter as follows: the standby power consumption of power adapter below 10 W can not exceed 0.3 W from Jul. 1, 2006; the standby power consumption of power adapter above 10 W but below 250 W can not be more than 0.5 W.
However, the standby power consumption of most power-frequency adapter is too much and far away from the standard of “Energy Star” Certification, which has become a technological problem.